1. Field of the Invention
This invention relates to a backlight suitable for use typically with a liquid crystal display apparatus, a display apparatus which includes a backlight, and a light source controlling method for controlling lighting of a backlight.
2. Description of the Related Art
In a liquid crystal display apparatus, pixels themselves disposed on a display panel do not emit light. Therefore, a backlight is disposed on the back of the display panel such that the back of the display panel is illuminated by the backlight to display an image and so forth. Together with increase of the screen size of the liquid crystal display apparatus, the display area of the display panel tends to increase, and also the size of the backlight itself is increasing significantly.
FIGS. 8A and 8B show an example of a backlight disposed on the back of a display panel of a liquid crystal display apparatus in the past. More particularly, FIG. 8A shows the backlight as viewed from the front, and FIG. 8B shows the backlight as viewed from a side. Referring to FIGS. 8A and 8B, in the backlight shown, a cold cathode fluorescent lamp (CCFL) is used as a light emitting element. The backlight includes a light box 1, and a plurality of cold cathode fluorescent lamps 2 disposed in a vertical column in the light box 1 and extending horizontally. A reflection sheet 6 is disposed on the back side of the cold cathode fluorescent lamps 2. A diffusion plate 4 is disposed on the front of the light box 1 in which the cold cathode fluorescent lamps 2 are disposed. It is to be noted here that the side on which a display panel is disposed with respect to the backlight is referred to as front while the other side is referred to as back or rear. This similarly applies to the following description herein. Particularly, in FIG. 8B, the left side is the front side. The diffusion plate 4 is formed, for example, from an acrylic sheet or plate having a size substantially equal to the display area of the display panel and a predetermined thickness so that it may diffuse light. Further, a plurality of diffusion sheets 3 are disposed on the front of the diffusion plate 4. The diffusion sheets 3 may be formed from a thin film of a resin material having such a characteristic that, for example, it provides light with some directional property.
FIGS. 9A and 9B show another example of a configuration of a backlight where a partition plate is provided between adjacent ones of light sources. Particularly, FIG. 9A shows the backlight as viewed from the front while FIG. 9B shows the backlight as viewed from a side. Referring to FIGS. 9A and 9B, the backlight shown has a configuration same as that of the backlight of FIGS. 8A and 8B except the partition plate mentioned above. In particular, a partition plate 7 is disposed between adjacent ones of the plural cold cathode fluorescent lamps 2 disposed in the light box 1 so that light fluxes from the cold cathode fluorescent lamps 2 may be introduced to the diffusion plate 4 without being mixed with each other.
Where the configuration shown in FIGS. 9A and 9B is employed, blinking, that is, turning off, of some of the light sources can be performed in synchronism with a display image. In particular, where an image is displayed on a liquid crystal display apparatus in which a backlight is used, light emission control of the backlight called blinking is sometimes used in order to assure high moving picture responsibility. More particularly, the liquid crystal display panel temporarily enters a state in which the display state thereof is not fixed within a period within which a display signal is written into pixels disposed on the panel. This state is likely to be perceived by the user and deteriorates the picture quality of the display image, particularly the responsibility of moving pictures. Therefore, within a period within which a display signal is written into the pixels, the light source of the backlight on the back side of a pertaining horizontal light is turned off so that no light is emitted from the light source in order to enhance the responsibility of moving pictures. Where the partition plates 7 are provided as seen in FIGS. 9A and 9B, light fluxes from adjacent ones of the light sources, that is, the cold cathode fluorescent lamps 2, do not mix with each other. Therefore, a blinking process can be performed appropriately. A particular example of lighting control where a blinking process is involved is hereinafter described in connection with preferred embodiments of the present invention.
Japanese Patent Laid-open No. 2003-50569 discloses a liquid crystal image display apparatus wherein lighting (turning on/off) control of a backlight is performed in synchronism with re-writing of an image in order to assure high moving picture visibility.
Incidentally, where light fluxes from light sources are partitioned by partition plates as in the backlight of FIGS. 9A and 9B, if the luminances of the light sources are not uniform, then the display image suffers from unevenness in luminance, resulting in deterioration of the picture quality. In particular, if the light fluxes from the cold cathode fluorescent lamps 2 which form the light sources have some unevenness in luminance or chromaticity, then this gives rise to unevenness in brightness as viewed from the front face side and provides unevenness in brightness of the display image. This results in deterioration of the picture quality of the display image. Such unevenness in luminance or chromaticity of emitted light is caused by a dispersion in luminance which the light sources, in the example described, cold cathode fluorescent lamps, originally have. The unevenness in luminance or chromaticity is sometimes caused also by the degree of progress of deterioration by a secular change.
One of possible solutions to the problem just described is to attach a photo-sensor in the proximity of each of light sources disposed in a backlight such that the luminance of light from the light source is corrected individually in response to the luminance or chromaticity detected by the photo-sensor. However, if a number of photo-sensors equal to the number of light sources are provided, then a great number of photo-sensors are demanded for one backlight. This provides a problem that the backlight is complicated very much in configuration.
It is to be noted that, although the problems where the backlight is configured principally for blinking are described above, the problem of unevenness in luminance of light emitted from light sources is involved also in such a configuration that no partition plate is provided as shown in FIGS. 8A and 8B.
Therefore, it is demanded to provide a backlight, a display apparatus and a light source controlling method by which light emission control free from display unevenness can be performed simply.